A conventional thermal image sensor consists of thermo piles arranged in a vertical direction, and scans this vertically-arranged sensor from left to right and from right to left in a constant cycle, thereby acquiring an entire thermal image of a room. At that time, the thermal image sensor acquires a thermal image of the entire room at the time of an initial startup, stores this thermal image as a background thermal image, and, after that, whenever the entire thermal image is newly acquired, calculates the difference between the entire thermal image and the background thermal image, and, when the difference value keeps a value equal to or greater than a human body detection threshold, determines that a person is existing at the corresponding pixel position (for example, patent reference 1).